Enhancement of the Electrode Activity of the Vanadium Redox Flow Battery by Higher Crystallinity of Carbon Matrix Using Seamless and Consecutive-porous Carbon Materials

The crystallinity of the carbon matrix and the surface oxygen groups of the electrode materials for vanadium redox flow batteries (VRFBs) are considered to be important for enhancing the activity of the electrochemical reactions. We applied seamless carbon materials with consecutive macropores as a novel electrode material for the VRFB. We heat-treated the seamless carbon materials from 1200 °C to 2200 °C in an Ar atmosphere, then oxidized them in air at the appropriate temperature. Although the number of surface oxygen groups, which are believed to be the active sites, decreased at the higher crystallinity of the carbon matrix, the electrode activity was simply increased at the higher crystallinity of the carbon matrix. This result suggests the increased π electron density enhanced the ion exchange between the active materials and protons at the active sites due to the higher p K a value. Next, we examined the necessity of the surface oxygen groups for the material by the thermal decomposition in the Ar atmosphere. The current density significantly decreased after the thermal decomposition of the surface oxygen groups. Hence, the surface oxygen groups are believed to be essential for the electrochemical reactions.